1. Technical Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine.
2. Related Art
In diesel engines and the like, a NOx occlusion-reduction catalyst (NOx catalyst, Lean NOx Trap [LNT]) may be disposed in an intermediate position in an exhaust pipe to remove nitrogen oxide (NOx) within the exhaust gas. In the LNT, the NOx is occluded in a normal, lean air/fuel mixture in the diesel engine. As a result of the mixture being periodically switched to a rich mixture, the occluded NOx is reduced to harmless nitrogen and discharged (which is referred to as “NOx reduction”).
As a method of forming the rich mixture, a method referred to as exhaust fuel addition is known. In the method, an additive fuel injector is provided in an exhaust pipe. Fuel is injected from the additive fuel injector into the exhaust pipe, thereby providing unburned fuel serving as a reducing agent to the LNT. Furthermore, when the LNT is regenerated (SOx reduction) after being poisoned by sulfur within the fuel, the fuel may be injected from the additive fuel injector to form the rich mixture.
However, in the exhaust fuel addition method, a problem occurs in that, as the additive fuel injector is used, the actual injection amount (i.e. true value) becomes less than the commanded value of the injection amount because of clogging caused by soot and the like. Various proposals have been made to overcome this problem. For example, in Japanese Patent Publication No. 3788314, a technology is disclosed for correcting the injection amount depending on an extent of temperature increase, when temperature rises as a result of the fuel being added.
However, the method in Japanese Patent Publication No. 3788314 is disadvantageous in that calculation of a correction value is relatively time-consuming because of effects of heat capacity of the catalyst.
As a method of correcting the injection amount, a method is known in which an A/F (air-to-fuel ratio) sensor measures an A/F value when the additive fuel injector is used to form a rich state. The correction value is obtained from the measured A/F value. However, in this method, as a result of the rich state being formed, a large amount of unburned hydrocarbon (HC) having large molecular size is present in the exhaust gas. The unburned HC may not sufficiently burn, even with catalytic influence. In general, it is also known that the A/F sensor tends to incorrectly measure the A/F value when a large amount of unburned HC is present.
When the fuel is continuously injected from the additive fuel injector over a long duration, the unburned HC may pass through the catalyst, resulting in poor emission. Therefore, the fuel can be continuously injected from the additive fuel injector for about only one or two seconds. However, when the fuel is injected from the additive fuel injector for a short time, the A/F sensor, which is commonly slow to respond, cannot accurately measure the A/F value.
Moreover, when the fuel is injected from the additive fuel injector for a short time, a problem also occurs in that the fuel is deposited on the wall surface. In this phenomenon, the fuel injected from the additive fuel injector is deposited onto the wall surface of the exhaust pipe near the additive fuel injector. Because of the deposition of fuel onto the wall surface, the actual A/F value becomes greater than an A/F value expected from the injection amount. To suppress the effect of the deposition onto the wall surface, the amount of fuel deposited onto the wall surfaces and the amount of deposited fuel which evaporates are required to be balanced. To balance the amounts, the fuel is required to be continuously injected from the additive fuel injector. The above-described problems are present in the method of calculating the correction value of the injection amount through measurement of the A/F after the fuel is injected from the additive fuel injector.